This document provides information about flow control and error control techniques in data link layer protocols. It discusses three mechanisms: stop-and-wait, go-back-N ARQ, and selective-repeat ARQ. Stop-and-wait flow control involves a sender keeping a copy of the last frame until receiving an acknowledgement and resending any unacknowledged frames. Cyclic redundancy check (CRC) is described as an error detection code used to detect accidental changes to data by dividing the data and CRC by a generator polynomial. The document also includes an explanation of how CRC works at the sender and receiver.

1. CSE350
Data Communications
East West University

2. PRESENTATION ON
Flow control and error control techniques in the
data link layer protocol
Presented by:
Md. Saiyadul Ferdous
ID: 2015-2-60-047
Md. Mohaiminul Hasan
ID: 2015-2-60-084

3. Data Link Layer
Data Link = Line Discipline + Flow Control + Error Control

4. Flow Control
 Flow control coordinates the amount of data that can be sent
before receiving acknowledgement.
 Flow control is a set of procedures that tells the sender how
much data it can transmit before it must wait for an
acknowledgement from the receiver.

5. Error Control
Error control includes both error detection and error
correction.
It allows the receiver to inform the sender if a frame
is lost or damaged during transmission and
coordinates the retransmission of those frames by
the sender.

6. Error and Flow Control Mechanisms
Stop-and-Wait
Go-Back-N ARQ
Selective-Repeat ARQ

7. Stop and Wait

8. Stop and Wait
 keeps a copy of the last frame until it receives an cknowledgement.
 For identification, both data frames and acknowledgements (ACK) frames
are numbered alternatively 0 and 1.
 Sender has a control variable (S) that holds the number of the recently
sent frame. (0 or 1)
 Sender starts a timer when it sends a frame. If an ACK is not received
within a allocated time period, the sender assumes that the frame was lost
or damaged and resends it

9. Cyclic redundancy check
 A cyclic redundancy check (CRC) is an error-detecting code commonly
used in digital networks and storage devices to detect accidental
changes to raw data.
 Cyclic codes are special linear block codes with one extra property. In a
cyclic code, if a code word is cyclically shifted (rotated), the result is
another code word.

10. CRC sender and receiver

11. EXPLAINED STEP BY STEP
 First of all, we should know the major rule of CRC which is: The Power of
each term shows the position of the bit, and the coefficient shows the
value of the bit

12. AT THE SENDER
 Step #1: At sender side, a string of n 0s is appended to the data unit to be
transmitted where n is less than the number of bits in the CRC generator.
 Step #2: The newly formed data unit is divided by the divisor using binary
division and reminder is obtained this reminder is called CRC, and when we
divide we use XOR Gate.
 Step #3: String on n 0s appended to the data unit earlier is replaced by the CRC
reminder –which us also n bit.
 Step #4: (Data unit + CRC) is transmitted to the receiver

13. AT THE RECEIVER
 Step #1: The receiver on receiving the codeword, divides (Data
unit + CRC) by the same division and checks the reminder.
 Step #2: The receiver checks if the reminder is zero.
 If Yes, Receiver assume that there is no error in data and and therefore accept
it.
 If No, There is an error in data and therefore reject it.

14. Thank You